US 2010/0030460A1 corresponding to JP 2008-249798A discloses a data distribution apparatus for distributing map differential data to a vehicle navigation apparatus when a map update for updating (e.g., adding or deleting) a map element (e.g., roads) of map data occurs. The map differential data is a difference between pre-update map data and post-update map data.
In the conventional data distribution apparatus, the map differential data is produced in units of blocks in a map and stacked in chronological order. Assuming that the map differential data is managed by versions, not only map differential data for the latest version of map data but also map differential data for the immediately previous version of map data are required to update the map data to the latest version. Accordingly, the volume of the map differential data distributed to the vehicle navigation apparatus is increased so that communication cost and time necessary to update the map data can be increased. Further, when a road is located over adjacent blocks, both of the blocks need to be updated. If only one block is updated without updating the other block, a road network is not ensured. That is, the road is disconnected in a map displayed on the vehicle navigation apparatus.